Shoe soles should primarily meet two requirements. First, they should provide good friction with the ground. Second, they should sufficiently cushion the ground reaction forces arising during a step cycle to reduce the strains on the wearer's muscles and bones. These ground reaction forces can be classified into three mutually orthogonal components, i.e., a component occurring in each of the X-direction, the Y-direction, and the Z-direction. The Z-direction designates a dimension essentially perpendicular (or vertical) to the ground surface. The Y-direction designates a dimension essentially parallel to a longitudinal axis of a foot and essentially horizontal relative to the ground surface. The X-direction designates a dimension essentially perpendicular to the longitudinal axis of the foot and essentially horizontal relative to the ground surface.
The largest ground reaction force component typically occurs in the Z-direction. Studies have shown that peak forces of approximately 2000 N may occur in the Z-direction during running. This value is about 2.5 to 3 times the body weight of a typical runner. Accordingly, in the past, the greatest attention was directed to the strains of the muscles and the bones caused by this force component and the many different arrangements for optimizing the cushioning properties of a shoe in the Z-direction.
Ground reaction forces, however, further include noticeable force components in the X-direction and in the Y-direction. Measurements have shown that forces of approximately 50 N in the X-direction and of approximately 250 N in the Y-direction may occur in a heel area during running. During other sports, for example lateral sports such as basketball or tennis, forces of up to 1000 N may occur in a forefoot area in the X-direction during side cuts, impact, and push off.
The aforementioned horizontal forces in the X- and Y-directions are one reason why running on an asphalt road is considered uncomfortable. When the shoe contacts the ground, its horizontal movement is essentially completely stopped within a fraction of a second. In this situation, the horizontally effective forces, i.e., the horizontal transfer of momentum, are very large. This is in contrast to running on a soft forest ground, where the deceleration is distributed over a longer time period due to the reduced friction of the ground. The high transfer of momentum can cause premature fatigue of the joints and the muscles and may, in the worst case, even be the reason for injuries.
Further, many runners contact the ground with the heel first. If viewed from the side, the longitudinal axis of the foot is slightly inclined with respect to the ground surface (i.e., dorsal flexion occurs). As a result, a torque, which cannot be sufficiently cushioned by compression of a sole material in the Z-direction alone, is exerted on the foot during first ground contact. This problem becomes worse when the runner runs on a downhill path, since the angle between the shoe sole and the ground increases in such a situation.
In addition, road surfaces are typically cambered for better water drainage. This leads to a further angle between the sole surface and the ground plane. Additional loads, caused by a torque on the joints and the muscles, are, therefore, created during ground contact with the heel. With respect to this strain, the compression of the sole materials in the Z-direction alone again fails to provide sufficient cushioning. Furthermore, during trail running on soft forest ground, roots or similar bumps in the ground force the foot during ground contact into an anatomically adverse inclined orientation. This situation leads to peak loads on the joints.
There have been approaches in the field to effectively cushion loads that are not exclusively acting in the Z-direction. For example, International Publication No. WO98/07343, the disclosure of which is hereby incorporated herein by reference in its entirety, discloses 3D-deformation elements that allow for a shift of the overall shoe sole with respect to a ground contacting surface. This is achieved by a shearing motion of an elastic chamber, where the walls are bent to one side in parallel so that the chamber has a parallelogram-like cross-section, instead of its original rectangular cross-section, under a horizontal load.
A similar approach can be found in U.S. Pat. No. 6,115,943, the disclosure of which is hereby incorporated herein by reference in its entirety. Two plates interconnected by means of a rigid linkage below the heel are shifted with respect to each other. The kinematics are similar to International Publication No. WO98/07343, i.e., the volume defined by the upper and lower plate, which is filled by a cushioning material, has an approximately rectangular cross-section in the starting configuration, but is transformed into an increasingly thin parallelogram under increasing deformation.
One disadvantage of such constructions is that cushioning is only possible along a single path, as predetermined by the mechanical elements. For example, the heel unit disclosed in U.S. Pat. No. 6,115,943 allows only a deflection in the Y-direction, which is simultaneously coupled to a certain deflection in the Z-direction. With respect to forces acting in the X-direction, the sole is substantially rigid. Another disadvantage of such constructions is that the horizontal cushioning is not decoupled from the cushioning in the Z-direction. Modifications of the material or design parameters for the Z-direction can have side effects on the horizontal directions and vice versa. Accordingly, the complex multi-dimensional loads occurring during the first ground contact with the heel, in particular in the above discussed situations with inclined road surfaces, cannot be sufficiently controlled.
Further, U.S. Pat. No. 5,224,810, the disclosure of which is also hereby incorporated herein by reference in its entirety, discloses dividing the overall sole of a shoe into two wedge-like halves which are shifted with respect to each other, wherein the movement is limited to the X-direction by means of corresponding ribs. Cushioning for ground reaction forces acting in the longitudinal direction (i.e. the Y-direction) of the shoe is not disclosed. In particular, the system does not provide any cushioning during ground contact with the heel.
It is, therefore, an object of the present invention to provide a cushioning element for a shoe sole that reduces loads on the muscles and the bones caused by multi-dimensional ground reaction forces, in particular during the first ground contact with the heel, thereby overcoming the above discussed disadvantages of the prior art.